This invention relates to a washer fluid monitor for a vehicle window washing system, the monitor providing a warning of low washer fluid in a washer fluid reservoir. It is particularly useful for such a system in a vehicle including a DC electrical supply and in which the window washing system includes an electric washer pump motor and/or additional electrically conducting apparatus grounding the washer fluid in the washer fluid reservoir to the DC electrical supply.
A problem with such systems in the prior art occurs at the interface between the electrical monitor and pumping systems and the washer fluid in the reservoir. In many such systems, the monitor probes are connected to different DC voltages from each other and/or from the fluid and thus tend to support an electrolytic action in the washer fluid with a flow of ions which tends to degrade the probes.
This electrolytic action may be reduced or eliminated if both probes and the washer fluid are maintained at substantially the same voltage. A system designed for this purpose is shown in the U.S. Pat No. 4,110,740 to Akita et al, issued Aug. 29, 1978. The system of this patent comprises an oscillator with the probes providing an impedance therebetween which determines the oscillation of the oscillator. The oscillator circuit is arranged so that it oscillates only when there is a high impedance between the probes--that is, when at least one of the probes is out of the fluid. When both probes are in the fluid and the impedance therebetween is low, the oscillator does not oscillate; and both probes are DC grounded. There is thus claimed to be little or no probe degrading ion current flow between the probes. However, the circuit of Akita et al is more expensive to build than is desired, since the main probe is DC isolated from the oscillator circuit with a transformer.